Typically, memory modules support an error correction code (ECC) by including an additional memory device on the module to store ECC bits. The term “ECC device” refers to the additional memory device that stores ECC bits. The term “data device” refers to the memory devices that store data rather than ECC bits. In general, the ECC device has the same density as the memory devices on the module. The term “density” refers to the storage capacity of a memory device.
In some cases, a significant fraction of the storage capacity of the ECC device is wasted. For example, the amount of memory that is wasted for a single sided dual inline memory module (DIMM) using x16 devices with a density of 1 Gbit is 64 Mbytes (or ½ of the capacity of a 128 Mbyte device). Table 1 illustrates the amount of wasted capacity of an ECC device for a 72 bit wide single sided DIMM using devices with various bit widths.
TABLE 1ECCECCDIMM widthDevice width in# of devices + #deviceMemoryin bitsbitsof ECC deviceswidthwasted72x8 8 + 1x8 No72x164 + 1x16Yes-½72x322 + 1x32Yes-¾th
As shown in Table 1, the amount of wasted capacity is significant. In addition, the amount of wasted capacity doubles as the density of the memory devices doubles.